Neurone

Original Editor - Lucinda hampton

Top Contributors - Lucinda hampton, Kim Jackson, Rucha Gadgil, Manisha Shrestha and Stacy Schiurring  

Introduction

Neurones are cells of the central nervous system, located within the grey matter, and responsible for all neurological functions of the brain.

Structure

Neurones vary in morphology and size substantially, but all share a number of features :

Cell body.

• Contains the nucleus and is the site of synthesis of virtually all neuronal proteins and membranes. Some proteins are synthesized in dendrites, but no proteins are made in axons and axon terminals, which do not contain ribosomes. Proteins and membranes that are required for renewal of the axon and nerve termini are synthesized in the cell body and assembled there into membranous vesicles or multiprotein particles. By a process called anterograde transport, these materials are transported along microtubules down the length of the axon to the terminals, where they are inserted into the plasma membrane or other organelles. Axonal microtubules also are the tracks along which damaged membranes and organelles move up the axon toward the cell body; this process is called retrograde transport. Lysosomes, where such material is degraded, are found only in the cell body.

• perikaryon: cytoplasm surrounding the nucleus

Axon

• generally the 'output' of the neurone
• Specialized for the conduction of a particular type of electric impulse, called an action potential, outward, away from the cell body toward the axon terminus. An action potential is a series of sudden changes in the voltage, or equivalently the electric potential, across the plasma membrane. When a neuron is in the resting (nonstimulated) state, the electric potential across the axonal membrane is approximately −60 mV (the inside negative relative to the outside); the magnitude of this resting potential is similar to that of the membrane potential in most non-neuronal cells. At the peak of an action potential, the membrane potential can be as much as +50 mV (inside positive), a net change of ≈110 mV. This depolarization of the membrane is followed by a rapid repolarization, returning the membrane potential to the resting value. These characteristics distinguish an action potential from other types of changes in electric potential across the plasma membrane and allow an action potential to move along an axon without diminution.
• a single process of variable length (even over a metre)
• terminates in terminal buttons which release neurotransmitters

Dendrites

• Generally the 'input' of the neurone
• Most neurons have multiple dendrites, which extend out-ward from the cell body and are specialized to receive chemical signals from the axon termini of other neurons. Dendrites convert these signals into small electric impulses and transmit them inward, in the direction of the cell body. Neuronal cell bodies can also form synapses and thus receive signals
• Particularly in the central nervous system, neurons have extremely long dendrites with complex branches. This allows them to form synapses with and receive signals from a large number of other neurons, perhaps up to a thousand.
• Electric disturbances generated in the dendrites or cell body spread to the axon hillock. If the electric disturbance there is great enough, an action potential will originate and will be actively conducted down the axon.

Types of neurones

Three main morphological patterns of neurones are described, depending on the relationship of the cell body to the axon and dendrites 1.

• multipolar
• bipolar
• pseudounipolar

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